The present invention relates generally to antennae loaded by one or more meanderlines (also referred to as variable impedance transmission lines or slow wave transmission lines), and specifically to a meanderline smart antenna providing adaptive operation in response to environmental stimuli.
It is generally known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements and the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna parameters, including input impedance, gain, directivity and the radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension must be on the order of a quarter wavelength of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wave length and half wave length antennae are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a significant need for physically smaller, less obtrusive, and more efficient antennae that are capable of operation in multiple frequency bands and/or in multiple modes (i.e., different radiation patterns). Smaller packages do not provide sufficient space for the conventional quarter and half wave length antenna elements. As is known to those skilled in the art, there is also an inverse relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennae. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennae capable of efficient multi-frequency and/or wide bandwidth operation. Finally, it is known that the relationship between the antenna frequency and the effective antenna length (in wavelengths) determines the antenna gain. That is, the antenna gain is constant for all quarter wavelength antennae of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter of a wavelength.
One basic antenna model commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi. A derivative of the half-wavelength dipole is the quarter-wavelength monopole antenna located above a ground plane. The physical antenna length is a quarter-wavelength, but with the ground plane the antenna performance resembles a half-wavelength dipole. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. Another conventional antenna is the patch, which provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wave length antenna, the patch antenna has a relatively low radiation efficiency.
Given the advantageous performance of quarter and half wavelength antennae, conventional antennae are typically constructed with elemental lengths on the order of a quarter wavelength of the radiating frequency. These dimensions allow the antenna to be easily excited and operated at or near a resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the resonant frequency decreases, the operative wavelength increases and the antenna element dimensions proportionally increase. The meanderline-loaded antenna (MLA) was developed to de-couple the conventional relationship between the antenna length and resonant frequency.
A typical meanderline-loaded antenna is disclosed in U.S. Pat. No. 5,790,080. A meanderline-loaded antenna is also known as a variable impedance transmission line (VITL) antenna. The antenna consists of two vertical conductive elements, a horizontal conductive element and a ground plane, with a gap separating each vertical conductive from the horizontal conductive element.
The antenna further comprises one or more meanderline variable impedance transmission lines bridging each gap. Each meanderline coupler is a slow wave transmission line structure carrying a traveling wave at a velocity less than the free space velocity. Thus the effective electrical length of the slow wave structure is considerably greater than it""s actual physical length. Consequently, smaller antenna elements can be employed to form an antenna having, for example, quarter-wavelength properties. Further, in one embodiment the slow wave structure includes separate switchable segments that can be inserted in and removed from the circuit with negligible losses. This switching action changes the effective electrical length of the meanderline coupler and thus changes the effective length of the antenna. Losses are minimized in the switching process because the meanderline is constructed with the active switching structure in the high impedance sections of the meanderline. Thus the current through the switching device is low, resulting in very low dissipation losses and a high antenna efficiency. Although the meanderline antenna offers desirable attributes with a smaller physical volume, as hand-held wireless communications devices continue to shrink, manufacturers continue to demand smaller antennae.
The meanderline-loaded antenna allows the physical antenna dimensions to be significantly reduced, while maintaining an effective electrical length that is a quarter wavelength multiple. The meanderline-loaded antennae operate in the region where the performance is limited by the Chu-Harrington relation, that is,
efficiency=FV2Q,
where:
Q=quality factor
V=volume of the structure in cubic wavelengths
F=geometric form factor (F=64 for a cube or a sphere)
Meanderline-loaded antennae achieve this efficiency limit of the Chu-Harrington relation while allowing the effective antenna length to be less than a quarter wavelength at the resonant frequency. Dimension reductions of 10 to 1 can be achieved over a quarter wavelength monopole antenna, while achieving a comparable gain.
All antennae, including the relatively physically small meanderline-loaded antenna, whether enclosed within or protruding from today""s popular handheld personal communications devices exhibit the so-called xe2x80x9chandxe2x80x9d or xe2x80x9cbodyxe2x80x9d effect. Although the antenna is designed and constructed to provide certain ideal performance characteristics, in fact, these characteristics are influenced, some significantly, by the proximity of near-field objects, such as a user""s hand, to the antenna while the communications device is in use. This effect is caused when the hand of a person or other grounded object, is placed close to the antenna, forming stray capacitances between the grounded object and the antenna. This effect can significantly detune the antenna, shifting the antenna resonant frequency either up or down, that is off-center with respect to the desired band of operation. The result is a reduction in the received or transmitted signal strength. Also, the hand effect can change the antenna radiation pattern in both the receive and transmit modes of operation. It is difficult to design an antenna that does not suffer the hand-effect problem, and furthermore, since each user handles and holds his or her personal communications device in a different orientation, there is no design strategy that can be universally employed to reduce or eliminate the hand effect.
The antenna constructed according to the teachings of the present invention is designed to overcome the hand effect by adaptively changing certain antenna dimensions in response to a change in one or more antenna performance parameters, thus reducing the hand effect and also any other performance effect that manifests itself by changing the measured antenna performance parameter.
The present invention is an antenna comprising a ground plane, one or more conductive elements, including a horizontal element and at least two spaced-apart vertical elements, each connected to the horizontal element by a meanderline coupler. The meanderline coupler has an effective length, as determined by its physical structure, that influences the total effective electrical length, operating characteristics and pattern shape of the antenna. The use of multiple vertical elements, each with its own meanderline coupler or the use of multiple meanderline couplers on a single vertical element provides controllable operation in multiple frequency bands. An antenna comprising meanderline couplers has a smaller physical size, yet exhibits comparable or enhanced performance over a conventional dipole antenna. Further, the operational bandwidth is greater than typically available from a patch antenna.
In one embodiment, a meanderline coupler antenna operates in two frequency bands, with a unique antenna pattern for each band (i.e., in one band the antenna has a an omnidirectional donut radiation pattern (referred to herein as the monopole mode) and in the other band the majority of the radiation is emitted in a hemispherical pattern (referred to as the loop mode). Advantageously, the meanderline-loaded antenna incorporates a piezoelectric material having changeable dimensional characteristics that in turn change certain antenna physical dimensions to improve antenna performance, especially to overcome the hand effect. A voltage generated in response to one more measured antenna parameters is applied to the piezoelectric devise for moving one or more of the antenna elements and thereby changing, i.e., improving, the measured performance parameter. When operative with a radio or receiver unit, this embodiment thus implements a dynamic radio/antenna feedback control loop that adaptively changes the antenna parameters in response to real time changes in the hand-effect.